.CIRCLE3 \ This gets called from CIRCLE2 below to calculate the \ line segments, which CIRCLE2 then sends to the I/O \ processor for drawing LDX #&FF \ Set FLAG = &FF to reset the ball line heap in the call STX FLAG \ to the BLINE routine below INX \ Set CNT = 0, our counter that goes up to 64, counting STX CNT \ segments in our circle .PLL3 LDA CNT \ Set A = CNT JSR FMLTU2 \ Call FMLTU2 to calculate: \ \ A = K * sin(A) \ = K * sin(CNT) LDX #0 \ Set T = 0, so we have the following: STX T \ \ (T A) = K * sin(CNT) \ \ which is the x-coordinate of the circle for this count LDX CNT \ If CNT < 33 then jump to PL37, as this is the right CPX #33 \ half of the circle and the sign of the x-coordinate is BCC PL37 \ correct EOR #%11111111 \ This is the left half of the circle, so we want to ADC #0 \ flip the sign of the x-coordinate in (T A) using two's TAX \ complement, so we start with the low byte and store it \ in X (the ADC adds 1 as we know the C flag is set) LDA #&FF \ And then we flip the high byte in T ADC #0 STA T TXA \ Finally, we restore the low byte from X, so we have \ now negated the x-coordinate in (T A) CLC \ Clear the C flag so we can do some more addition below .PL37 ADC K3 \ We now calculate the following: STA K6 \ \ K6(1 0) = (T A) + K3(1 0) \ \ to add the coordinates of the centre to our circle \ point, starting with the low bytes LDA K3+1 \ And then doing the high bytes, so we now have: ADC T \ STA K6+1 \ K6(1 0) = K * sin(CNT) + K3(1 0) \ \ which is the result we want for the x-coordinate LDA CNT \ Set A = CNT + 16 CLC ADC #16 JSR FMLTU2 \ Call FMLTU2 to calculate: \ \ A = K * sin(A) \ = K * sin(CNT + 16) \ = K * cos(CNT) TAX \ Set X = A \ = K * cos(CNT) LDA #0 \ Set T = 0, so we have the following: STA T \ \ (T X) = K * cos(CNT) \ \ which is the y-coordinate of the circle for this count LDA CNT \ Set A = (CNT + 15) mod 64 ADC #15 AND #63 CMP #33 \ If A < 33 (i.e. CNT is 0-16 or 48-64) then jump to BCC PL38 \ PL38, as this is the bottom half of the circle and the \ sign of the y-coordinate is correct TXA \ This is the top half of the circle, so we want to EOR #%11111111 \ flip the sign of the y-coordinate in (T X) using two's ADC #0 \ complement, so we start with the low byte in X (the TAX \ ADC adds 1 as we know the C flag is set) LDA #&FF \ And then we flip the high byte in T, so we have ADC #0 \ now negated the y-coordinate in (T X) STA T CLC \ Clear the C flag so we can do some more addition below .PL38 JSR BLINE \ Call BLINE to draw this segment, which also increases \ CNT by STP, the step size CMP #65 \ If CNT >= 65 then skip the next instruction BCS P%+5 JMP PLL3 \ Jump back for the next segment CLC \ Clear the C flag to indicate success RTS \ Return from the subroutine .CIRCLE2 \ This is the entry point for this subroutine STZ LSP \ Reset the ball line heap by setting the ball line heap \ pointer to 0 JSR CIRCLE3 \ Call CIRCLE3 to populate the ball line heap \ Fall through into LS2FL to send the ball line heap to \ the I/O processor for drawing on-screenName: CIRCLE2 [Show more] Type: Subroutine Category: Drawing circles Summary: Draw a circle (for the planet or chart) Deep dive: Drawing circlesContext: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: This subroutine is called as follows: * HFS2 calls CIRCLE2 * TT128 calls CIRCLE2

Draw a circle with the centre at (K3, K4) and radius K. Used to draw the planet and the chart circles. Arguments: STP The step size for the circle K The circle's radius K3(1 0) Pixel x-coordinate of the centre of the circle K4(1 0) Pixel y-coordinate of the centre of the circle Returns: C flag The C flag is cleared Other entry points: CIRCLE3 Just add the circle segments to the existing ball line heap - do not send the send the ball line heap to the I/O processor for drawing on-screen

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Subroutine BLINE (category: Drawing circles)

Draw a circle segment and add it to the ball line heap

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Entry point CIRCLE3 in subroutine CIRCLE2 (category: Drawing circles)

Just add the circle segments to the existing ball line heap - do not send the send the ball line heap to the I/O processor for drawing on-screen

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Subroutine FMLTU2 (category: Maths (Arithmetic))

Calculate A = K * sin(A)

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The ball line heap pointer, which contains the number of the first free byte after the end of the LSX2 and LSY2 heaps (see the deep dive on The ball line heap for details)

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Label PL37 is local to this routine

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Label PL38 is local to this routine

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Label PLL3 is local to this routine